What does passive transport refer to?

Passive transport refers to the movement of molecules across a cell membrane without the expenditure of energy. This process occurs along the concentration gradient, meaning that molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. Since it relies on natural kinetic energy and does not require energy input from the cell, it is termed "passive."

In biological systems, passive transport can occur through several mechanisms, including diffusion, facilitated diffusion, and osmosis, each of which allows specific substances to cross membranes efficiently without the cell having to use metabolic energy. This characteristic is essential for maintaining homeostasis within cells, allowing them to take in necessary substances and expel waste products.

In contrast, other options like the movement of molecules with energy, the active movement of ions, or transport through vesicles all involve energy expenditure or specific cellular processes that require ATP or another energy source, which distinguishes them from the passive transport mechanism.